There's actually a "soft preview" service that runs on Friday afternoons from Moorgate to Gordon Hill. It leaves Moorgate around 1430. From what I've seen it runs non-stop from Finsbury Park to Gordon Hill.

From what I've gleaned from the train geeks on twitter, there are now 3 trains that are running limited services (Moorgate to Gordon Hill) and they will be brought into full service on 28 January. The remaining units will then be phased in over the next 3 months.

I've been on the new train from Moorgate to Gordon Hill this week - they run through the day off-peak. More seats and more room for standing, so in theory, there should be a little less "can you move down please" once they've launched.